Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These example systems operate in unlicensed spectrum.
Recently, the use of device-to-device (D2D) communications as an underlay to cellular networks has been proposed as a means to take advantage of the proximity of wireless devices operating within the network, while also allowing devices to operate in a controlled interference environment. In one suggested approach, D2D communications share the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D communications use. However, dynamic sharing of the cellular spectrum between cellular services and D2D communications is a more likely alternative than dedicated reservation, because cellular spectrum resources are inherently scarce and because dynamic allocation provides greater network flexibility and higher spectrum efficiency.
The Third Generation Partnership Project (3GPP) refers to Network Controlled D2D as “Proximity Services” or ProSe, and efforts aimed at integrated D2D functionality into the Long Term Evolution (LTE) specifications are underway. The ProSe Study Item (SI) recommends supporting D2D operation between wireless devices—referred to as user equipments or UEs by the 3GPP—that are out of network coverage, and between in-coverage and out-of-coverage wireless devices. In such cases, certain UEs may regularly transmit synchronization signals to provide local synchronization to neighboring wireless devices.
The ProSe SI also recommends supporting inter-cell D2D scenarios, where UEs camping on possibly unsynchronized cells are able to synchronize to each other. Still further, the ProSe SI recommends that in the LTE context, D2D-capable UEs will use uplink (UL) spectrum for D2D communications, for Frequency Division Duplex (FDD) cellular spectrum, and will use UL subframes from Time Division Duplex (TDD) cellular spectrum. Consequently, the D2D-capable UE is not expected to transmit D2D synchronization signals—denoted as D2DSS—in the downlink (DL) portion of the cellular spectrum. That restriction contrasts with network radio nodes or base stations, referred to as eNodeBs or eNBs in the 3GPP LTE context, which periodically transmit Primary Synchronization Signals, PSS, and Secondary Synchronization Signals, SSS, on the downlink.
The PSS/SSS enable UEs to perform cell search operations and to acquire initial synchronization with the cellular network. The PSS/SSS are generated based on pre-defined sequences with good correlation properties, in order to limit inter-cell interference, minimize cell identification errors and obtain reliable synchronization. In total, 504 combinations of PSS/SSS sequences are defined in LTE and are mapped to as many cell IDs. UEs that successfully detect and identify a sync signal are thus able to identify the corresponding cell-ID, too.
To better appreciate the PSS/SSS configurations used by eNBs on the DL in LTE networks, FIG. 1 illustrates time positions for PSS and SSS in the case of FDD and TDD spectrums. FIG. 2 illustrates PSS generation and the resulting signal structure, FIG. 3 illustrates SSS generation and the resulting signal structure.
FIG. 2 particularly highlights the formation of PSS using Zadoff-Chu sequences. These codes have zero cyclic autocorrelation at all nonzero lags. Therefore, when a Zadoff-Chu sequence is used as a synchronization code, the greatest correlation is seen at zero lag—i.e., when the ideal sequence and the received sequence are synchronized. FIG. 3 illustrates SSS generation and the resulting signal structure. In LTE, the PSS as transmitted by an eNB on the downlink is mapped into the first 31 subcarriers on either side of the DC subcarrier, meaning that the PSS uses six resource blocks, with five reserved subcarriers on each side, as shown in the following figure. Effectively, the PSS is mapped on to the middle 62 subcarriers of the OFDM resource grid at given symbol times, where “OFDM” denotes Orthogonal Frequency Division Multiplexing, in which an overall OFDM signal comprises a plurality of individual subcarriers spaced apart in frequency and where each subcarrier at each OFDM symbol time constitutes one resource element.
As FIG. 3 illustrates, the SSS are generated not using Zadoff-Chu sequences, but rather using M sequences, which are pseudorandom binary sequences generated by cycling through each possible state of a shift register. The shift register length defines the sequence length. SSS generation in LTE currently relies on M-sequences of length 31.
With the above in mind, the following equation defines the physical cell identifier of a given cell in an LTE network,NIDCELL=3NID(1)+NID(2),where NID(1) is the physical layer cell identity group (0 to 167), and where NID(2) is the identity within the group (0 to 2). As noted, this arrangement defines a cell identifier space of 504 values. The PSS is linked to the cell identity within the group NID(2), while the SSS is linked to the cell identity within the group NID(1) and the cell identity within the group NID(2). In particular, the PSS is a Zadoff-Chu sequence of complex symbols having length-62. There are three root sequences, indexed by the cell identity within the group NID(2). As for the SSS, two length-31 sequences are scrambled as a function of the cell identity from the group NID(1) and from the group NID(2). A receiver obtains the cell identity conveyed by the PSS and SSS by demodulating the PSS to obtain the value within the group NID(2) and then uses that knowledge to demodulate the SSS to obtain the value within the group NID(1).
Because of the desirable properties of the Zadoff-Chu and M sequences used to generate the PSS and SSS in LTE, and because of the preexisting investment in algorithms and associated device-side processing as just outlined, there is an express interest in reusing these “legacy” PSS/SSS signal generation and detection techniques for D2D Synchronization Signals, D2DSS. Further aspects of D2DSS were considered at the TSG RAN1 #74bis meeting of the Technical Specifications Group or TSG responsible for the Radio Access Network (RAN) in 3GPP. TSG RAN is responsible for defining the functions, requirements and interfaces of the Universal Terrestrial Radio Access Network (UTRAN) and the Evolved UTRAN (E-UTRAN), for both FDD and TDD modes of operation. The following working assumptions were set forth in the meeting:
Synchronization sources transmit at least a D2DSS: D2D Synchronization Signal                a. May be used by D2D UEs at least to derive time/frequency        b. May (FFS) also carry the identity and/or type of the synchronization source(s)        c. Comprises at least a PD2DSS                    i. PD2DSS is a ZC sequence            ii. Length FFS                        d. May also comprise a SD2DSS                    iii. SD2DSS is an M sequence            iv. Length FFS                        
As a concept for the purpose of further discussion, without implying that such a channel will be defined, consider a Physical D2D Synchronization Channel or PD2DSCH:                e. May carry information including one or more of the following (For Further Study or FFS):                    i. Identity of synchronization source            ii. Type of synchronization source            iii. Resource allocation for data and/or control signaling            iv. Data            v. others FFS                        
A synchronization source is any node transmitting D2DSS                f. A synchronization source has a physical identity PSSID        g. If the synchronization source is an eNB the D2DSS is Rel-8 PSS/SSS        h. Note: in RAN1#73, “synchronization reference” therefore means the synchronization signal(s) to which T1 relates, transmitted by one or more synchronization source(s).        
Even though a range of different distributed synchronization protocols are possible, one option under consideration by the 3GPP is based on hierarchical synchronization with the possibility of multi-hop sync-relay. In short, some nodes adopt the role of synchronization masters—sometimes referred to as Synchronization Heads (SH) or as Cluster Heads (CH)—according to a distributed synchronization algorithm. If the synchronization master is a UE, it provides synchronization by transmitting D2DSS and/or PD2DSCH. If the synchronization master is an eNB it provides synchronization by PSS/SSS and broadcast control information, such as being sent using MIB/SIB signaling, where MIB denotes “Master Information Block” and SIB denotes “System Information Block.”
The synchronization master is a special case of synchronization source that acts as an independent synchronization source, i.e., it does not inherit synchronization from other nodes by use of the radio interface. UEs that are under coverage of a synchronization source may, according to predefined rules, transmit D2DSS and/or PD2DSCH themselves, according to the synchronization reference received by their synchronization source. They may also transmit at least parts of the control information received from the synchronization master by use of D2DSS and/or PD2DSCH. Such a mode of operation is referred to herein as “sync-relay” or “CP-relay.”
It is also helpful to define a “synchronization reference” as a time and/or frequency reference associated with a certain synchronization signal. For example, a relayed synchronization signal is associated with the same synchronization reference as the sync signal in the first hop.
A number of advantages or benefits flow from reusing legacy PSS/SSS for D2DSS. sync signals. For example, because UEs must already detect and process PSS/SSS signals transmitted from eNBs in the network, substantially the same algorithms and processing can be reused for detecting D2DSS if the same PSS/SSS sequences are used for D2DSS. However, it is recognized herein that a number of potential issues arise with such reuse.
Consider, for example, the assumption that the cell-ID [0, . . . , 503] identifies a synchronization reference or source provided from an eNB operating in an LTE network. In a similar fashion, one assumes that a D2D identity will be used to identify a synchronization reference or source provided from a D2D-enabled UE. The D2D-identity may be significantly longer than the cell-ID, e.g., 16 bits or more, and it cannot be mapped to the D2DSS without significantly degrading sync detection performance.